This invention relates in general to an apparatus and method for the clarification of water where the removal of suspended particles is accomplished by a flocculating agent and flotation using a stream of rising, microscopic air bubbles. More specifically, it relates to an improved clarification apparatus and method of the general type described in U.S. Pat. Nos. 4,022,696 and 4,931,175 using single stage flotation in a flotation tank.
Water clarification, the removal of particulate contaminants suspended in water, is used to treat waste water from manufacturing processes, particularly in the paper and pulp industries, in the treatment of municipal water drinking supplies, and in sewage treatment. The water can be clarified by sedimentation or flotation of the particles. Known sedimentation techniques and apparatus are efficient, but are limited even at peak efficiency to a comparatively slow sedimentation rate, about 0.5 gallons per minute per square foot (40 l/m.sup.2 /min). To handle large volumes of raw input water, sedimentation facilities must therefore be large, with attendant cost and space utilization disadvantages.
Flotation techniques dissolve a few percent of air by volume in pressurized water and then release the air in the form of microscopic bubbles which attach to the particles and carry them upwardly to the surface where they form a floating sludge. The particles are usually flocculated using conventional flocculating agents such as alum before the air bubbles are introduced. Flotation techniques are theoretically capable of achieving clarification rates of 7.5 gallons per minute per square foot of flotation area (300 l/m.sup.2 /min). Heretofore in practice the rates have been less than this theoretical value, but significantly better than for sedimentation techniques.
Several early attempts by one of the applicants to use flotation techniques to clarify water are described in U.S. Pat. No. 2,874,842 issued in 1959 and U.S. Pat. No. 3,182,799 issued in 1965. They use a stationary tank with no skimmers or other moving components in the tank. The gas bubbles were introduced via the main raw water inlet and guided within the tank by an internal deflector ('842) or a stack of internal baffles ('799). Because the gas bubbles were guided by stationary components, there was no design problem created by the turbulence of moving parts in the flotation tank. Also these devices did not lend themselves to treatment at high flow rates. The '799 apparatus had the additional problems in that (i) the inlet water had to be separately fed from the side to the region between each adjacent pair of baffles and (ii) the flow paths for floated particles varies depending on the vertical position of the associated baffles defining the flow path. This latter situation means that the apparatus either does not fully treat the inlet water, or is slow.
One of the present applicants also holds several other U.S. patents for water clarification apparatus and processes, including U.S. Pat. Nos. 4,022,696; 4,377,485; 4,626,345; 4,184,967; and 4,931,175, which greatly improve over the performance of the early '842 and '799. In the '696 clarifier, sold under the trade designations "SPC" and "Supracell", the flotation occurs in a circular tank. The raw water is fed into the tank via a central pipe, a hydraulic joint, and an inlet pipe with multiple outlets immersed in the tank which rotates about the tank. The inlet flow is oriented opposite to the direction of rotation of the inlet pipe and is at a velocity with respect to the rotation rate such that the raw water has a net zero velocity as it enters the tank. The raw water inlet flow assembly and a scoop for removing the floated sludge are mounted on a carriage that rotates about the tank. The scoop is preferably of the type described in U.S. Pat. No. 4,184,967 of one of the present applicants. The rate of rotation is set so that the floated particles will reach the surface of the water held in the tank in the time of one rotation. A good degree of clarification can be achieved with each rotation of the carriage using a comparatively shallow tank, e.g. 16-18 inches. This SPC unit clarifies at a rate of about 130 liters/m.sup.2 /min. This is much better than the rate using sedimentation techniques, but less than half of the theoretical maximum rate.
In the Supracell and the other clarifiers described in the aforementioned patents, there are common design features and limitations. For one, the flotation of the flocced particles is generally vertical, and is in a body of water that is comparatively shallow and generally free from turbulence. Also, in all of these designs the removal of the floated sludge is by a bladed, rotating scoop feeding an inclined discharge pipe. Also, heretofore, in order to increase the capacity of a given type of clarifier, one built a larger diameter tank. While in theory one can build a large enough tank to accommodate any clarification load, cost and space constraints have provided practical limitations on the capacities of these units.
It is therefore a principal object of this invention to provide an improved water clarification apparatus that can increase the clarification rate of known flotation-type clarifiers by a factor of at least two with no attendant increase in the size of the unit.
Another principal object is to provide the foregoing increase in productivity with a comparatively small increase in cost.
Yet another object is to provide an improved sludge removal apparatus.